Some systems use a signal obtained from the load supporting end of an actuator to supply pressure to the pilot valve in response to the failure of the engine. However these systems do not provide a float condition of operation. Other systems use a make-up valve which can be vented and thus opened to allow fluid to be passed to tank from a load supporting end of an actuator upon failure of the engine. However these systems do not provide an accurate control of the load during lowering because the check in the make-up may not respond quickly enough due to large amounts of fluid flowing therethrough.
A system, such as that in the U.S. Pat. No. 3,840,049 issued to Jesse L. Field, Jr. on Oct. 8, 1974 and assigned to the assignee of the present invention, teaches a system having a float condition and a mechanism for providing a lowering of a load under dead engine conditions, but this system also requires venting behind the make-up and having to control the degree of opening of the make-up during lowering. The system is also more complex in that it requires two different make-up valves and a selector valve responsive to pilot pressure to provide both float and dead engine lower.
The present invention is directed to overcoming one or more of the problems as set forth above.